While many biomedical electrodes are known in the art, they use water-containing electroconductive materials such as conductive creams, pastes, and gels that incorporate natural polymers such as karaya gum so as to provide good contact between the skin surface and the electrode and reduce electrical resistance across the skin-electrode interface. Such conductive materials are placed between the skin and the electrode plate so as to ensure good electrical connection of the skin surface to biomedical diagnostic equipment such as high-impedance electro-myographs and electrocardiographs. Conductive creams and pastes are unpleasant to use, are sloppy and will often foul the skin surface.
U.S. Pat. No. 4,125,110 teaches the use of natural polymers as conductive materials, but since they contain water as the major ingredient, provision must be made for preventing loss of water such as by evaporation in order to maintain the desired stable electrical characteristics prior to use. Water evaporation also occurs during use of the electrode and the flexibility of the gel, which contains natural polymers as the basic component, is reduced to such an extent that strong adhesion to the skin surface is not ensured. Because natural polymers originate in nature, there is a great inconsistency in the physical and chemical properties of natural polymers and in the amount of impurities present. Furthermore, the natural polymers which usually contain water support growth of microorganisms and have the potential for creating adverse skin sensitivities including antigen-antibody reactions and allergic reactions.
As a common problem associated with these conductive creams, pastes and natural polymer gels, they contain water as a conductive ingredient and require elaborate packaging to prevent loss of water prior to use. As the water content decreases, the electrical properties of the electrode are impaired, making them unsuitable for prolonged use on the skin which will dry out with time.
In order to overcome many problems associated with these "wet type" electrodes, biomedical electrodes have been proposed that utilize adhesives impregnated with conductive particles (e.g., metal particles) instead of water as the conductive material. These "dry type" electrodes have improved adhesion to the skin surface, can be securely attached to the skin and are free from the problems associated with the presence of water, but one great disadvantage results from the fact that such electrodes generally do not provide stable electrical signals.
It is speculated that dispersed conductive filler particles in the adhesive form a discontinuous, electrically conductive path which develops non-uniform electrical fields between particles, causing high signal/noise (S/N) ratios which are detrimental to the functions of biomedical electrodes.
Biomedical electrodes have also been proposed, which use, in place of the conductive natural polymers described above, polymers including monomeric units of carboxylic acid or salts thereof, or nonionic hydrophilic polymers including water-soluble monomeric units as disclosed in, for example, U.S. Pat. Nos. 4,066,078, 4,352,359 and 4,273,135, and such biomedical electrodes have proved to some extent satisfactory.